Recently, progress of powder metallurgy technology makes it possible to produce the complicated form in need of high dimensional accuracy into near-net-form. And the products that are manufactured by making use of the powder metallurgy technology becomes applicable to a variety of technological fields.
The iron-based mixed powder for powder metallurgy is produced as follows. At first, iron-based mixed powder, which is produced by mixing a powder for an alloy and a lubricant with iron-based powder, is filled up in a die cavity. Here, the powder for an alloy is one such as copper powder or graphite powder. The lubricant is one such as zinc stearate or lithium stearate. Second, they are pressurized to be formed, and subsequently they are subjected to sintering process to become a sintered body. Third, in accordance with the necessity, they are fabricated by cutting to be a final product.
Thus, the sintered body that is manufactured in such a way has a high porosity. At the moment, the sintered body has a high cutting-resistance (cutting-force), compared with metal materials produced by the melting method such as wrought steel and cast iron. For such a reason, it has conventionally been done to add a various sorts of powder such as Pb, Se, Te, Mn, S and so forth to iron-based powder, or to add such elements to the iron-based powder by alloying in an atomizing process. Such treatment has been done in order to improve the machinability of a sintered body.
However, since the melting point of Pb is as low as 330 degrees C., Pb melts in the midway of the sintering process. At the same moment, since Pb is not soluble into the ferrite, a problem happens that it is difficult to carry out uniform distribution of Pb all over a matrix. Moreover, because Se and Te embrittle the sintered body, a problem happens that degrading the mechanical property of the sintered body is significant.
As for adding the powder, in order to improve machinability, it is proposed that a variety of powders other than the above-mentioned ones are added.
As one example, addition of inorganic compound powder of high hardness to the iron-based mixed powder, as a chipping promotion material, is proposed. In this method, particles of the inorganic compound powder become a concentrating points of stress, when a part to be cut carries out plastic deformation at the time of cutting, and enforce the cut-scraps to be a small size, thereby reduce the contact surface area between a cutting tool and scraps to lower frictional resistance, and thereby prevent tool wear.
For example, in the Japanese Unexamined Patent Application Publication No. 61-147801, it is proposed to mix 0.05 to 5 mass % of manganese sulfide (MnS) powder of 10 micrometers or less in size into iron powder. Further, the Japanese Examined Patent Publication No. 46-39654 proposes a method for preparing a chipping promotion material, which means, adding BaSO4 or BaS independently or in combination. And furthermore, in the Japanese Unexamined Patent Application Publication No. 2002-155301, fluorides of alkaline earth metals such as CaF2, MgF2, SrF, and BaF2are proposed. Similarly, in the Japanese Patent No. 3073526, addition of the molten mixture of CaF2 and BaF2, or the combination of MnS and molten mixture of CaF2 and BaF2 is proposed.
Although such addition of chipping promotion material reduces the contact surface area between a cutting tool and scraps like the above and it is effective on lowering the frictional resistance, there is no function, which protects the tool surface, such as suppressing oxidization caused by frictional heat. (Here, the frictional heat generates on cutting.) And since an intermittent impact is further given to a tool at an intermittent collision between the tool and surfaces forming pores in a sintered body, there is a problem of quality-of-the-material degradation by oxidization on the surface of a tool, or a chip or fracture of the tool by generating the fine sized crack inside the tool by the intermittence impact.
As a method of preventing deterioration of the tool surface under cutting typified by oxidization, it is proposed to distribute the ceramics of the low melting point beforehand in a work material, to soften the ceramic particles exposed to the working side at the time of cutting, by the frictional heat, and to make the ceramic particles adhere and spread on the tool surface, and to make a tool protective film, what is called, an overlaying layer form. For example, Japanese Unexamined Patent Application Publication No. 9(1997)-279204 proposes such an iron-based mixed powder for powder metallurgy, which contains CaO—Al2O3—SiO2 based compound oxide powder of 0.02 to 0.3 mass % as the ceramic powder of low melting point, which has anorthite-phase and/or gehlenite-phase, and whose average particle diameter is 50 micrometers or less, in an iron powder as main ingredient.
However, there is a problem that in some cutting conditions, generation of frictional heat between tool and a work material is insufficient, and the low fusing point ceramics does not become soft, and therefore tool protective film is not formed.